Polymeric foams can be generally classified as either closed-cell foams or as open-cell foams. Open-cell foams can be used as a matrix to contain various liquids. They are capable of various industrial applications such as, for example, use in wipes and diapers, as carriers and ion exchange resins. For some of these applications, it is desirable to have porous crosslinked polymer blocks which have a very low density and a high capacity of absorbing and retaining liquids. Such high absorption capacity, low density, porous polymer blocks can be prepared by polymerizing a specific type of water-in-oil emulsion known as high internal phase emulsion (HIPE) having relatively small amounts of a continuous oil phase and relatively greater amounts of an internal water phase.
Such high absorption capacity, low density foams are prepared in U.S. Pat. No. 4,522,953 by polymerizing and crosslinking the monomers in the continuous oil phase of a high internal phase water-in-oil emulsion with a polymerization initiator such as potassium persulfate. Generally, these high internal phase water-in-oil emulsions contain at least 90 weight percent of an aqueous liquid as the internal phase. In order to obtain this high internal phase water-in-oil emulsion, a surfactant must be used to stabilize the emulsion.
One class of surfactants used to produce hydrophilic foams by such processes are sorbitan fatty acid esters. Commercial sorbitan fatty acid esters are a combination of mono-, di-, tri-, and tetra-fatty acid esters of sorbitan C.sub.6 (H.sub.2 O).sub.5 H.sub.2, as well as mono- and di-fatty acid esters of isosorbide C.sub.6 (H.sub.2 O).sub.4 H.sub.2 and polyol impurities.
Relatively large amounts of sorbitan fatty acid ester surfactants are required to prepare high internal phase emulsions. To stabilize an emulsion consisting of 30 parts aqueous phase dispersed in 1 part oil phase (by volume), a 10 percent by weight or greater concentration of sorbitan fatty acid ester surfactant in the oil phase is generally needed. Use of such a large amount of surfactant is undesirable since it can adversely affect foam performance by diluting the oil phase. The large quantity of surfactant also increases the raw material cost of producing a foam.
Another disadvantage of using commercially available sorbitan fatty acid esters as the emulsifier is that a water-soluble sludge forms when these surfactants are dissolved in the oil phase. It has been found that polyols such as free sorbitan and isosorbide are the main components of the sludge. The sludge must be removed before the surfactant-oil solution can be used to prepare an emulsion in order to prevent plugging of the process mixing equipment.
It is therefore an object of the present invention to provide a more effective surfactant for the preparation of curable high internal phase water-in-oil emulsions for producing low density crosslinked polymeric materials.